Ubiquitin ligase Cbl-b plays a crucial role in disuse-mediated muscle atrophy: Cbl-b caused ubiquitination and subsequent degradation of IRS-1, resulting in skeletal muscle atrophy. Since Cbl-b in skeletal muscle is preferentially expressed under unloading conditions, we hypothesized that elucidating the regulation pathway of Cbl-b expression leads to the clarification of the sensing mechanism for unloading stress. Here we report the mechanism of sensing for microgravity, using rat or mouse skeletal myotubes cultured under unloading conditions. Microgravity or simulated microgravity conditions significantly caused decrease in myotube diameter, indicating that skeletal muscle myotubes per se can sense microgravity stress. Furthermore, there was a potent oxidative stress responsive element from -110 to -60 bp of the Cbl-b promoter. We suggest that unloading stress induced muscle atrophy through oxidative stress-mediated Cbl-b expression and mitochondria dysfunction.